All known blood-pressure measuring instruments which used in practice and are non-invasive use the measuring method according to Riva-Rocci/Korotkoff (RR-method) or, instead of the Korotkoff microphone, the oscillation of the pressure in an elastic cuff in a modified way. This applies to instruments for manual use as well as to automated instruments which are offered as mobile blood pressure monitors measuring for 24 hours a day. The main disadvantages of blood pressure monitors of this kind, which one basically can also have on one's person, are that they are very unpleasant for the patient in the long run due to the repeated interruption of the blood circulation and that the consequently long time intervals at which the blood pressure is taken do not allow a continuous measurement.
A further known fact is that the speed of the pulse waves allows a certain access to the blood pressure. In measuring the speed of the pulse waves or pulse-wave running time (PWL) the running time of the pulse wave caused by every heartbeat is measured, wherein either the time difference between the R-peak of the electro-cardiogram (ECG) and the pulse's arrival at a peripheral artery is measured or the time difference between two pulses whose distance to the heart is different is detected by means of mechanical or optical sensors. The pulse-wave running time determined in this way correlates with mean blood pressure values to an intra-individually large extent. The main disadvantage of this method is that a separate measurement of diastolic and systolic pressure values is not possible on principle.
Furthermore, there are measuring instruments on the market which, using photoelectric means, continuously determine alterations in the blood quantity in the ear lobes, which varies with the pulse, but this determination is only qualitative and very approximate. These so-called ear pulse measuring instruments (OPM), however, which have basically been known for a long time, only serve to determine the pulse frequency.
Other proposals for the non-invasive, continuous measurement of blood pressure which work in that a sensor is placed unremovably and in an exact fashion over a discrete artery (e.g. brachial artery in the arm) practically fail because of their being much too sensitive to movement artefacts.